When More Is Less: Higher Magnetic Fields and Their Limited Impact on SNR per Time Unit of Acquisition Time in Single Voxel Spectroscopy

Magnetic resonance spectroscopy (MRS) offers significant diagnostic potential but is inherently constrained by low signal-to-noise ratio (SNR). While increasing the main magnetic field strength B_0 is theoretically linked to increased SNR, practically obtained gains in SNR from B07/4 to B0 depending on the domination of thermal noise at high B0, are not always realized. Especially in clinical settings the maximum reachable SNR is further constrained by the total available acquisition time (TA) and the regulatory limits on maximum tolerable specific absorption rate (SAR). This work attempts to derive mathematical expressions that enable systematical analysis of the theoretical achievable SNR-gain. One important notion is this context is the SNR gain per unit of measurement time as a function of the main magnetic field B0 strengths in the case of single voxel spectroscopy (SVS) pulse sequences. Our findings indicate that under given fixed total amount of (patient acceptable) measurement time TA, and maximum tolerable SAR limitation, together with conditions that ensure the adiabaticity of specific sequences, further increasing B0 does not further improve SNR per unit of measurement time. Key factors were identified, including RF-pulse bandwidth scaling with B0 and longitudinal relaxation time (T1) dependencies, impact the net gain as well. Our theoretical analysis emphasizes critical considerations for optimizing SNR per unit time in clinical MRS, even challenging the presumption that higher magnetic fields B0 always yield improved SNR per time unit of measurement time performance.